Fix nested parallel transition targets

.changeset/calm-machines-enter.md

@@ -0,0 +1,7 @@
+---
+"@typeonce/effect-machine": patch
+---
+
+Allow local and branch targets to enter inactive nested parallel states. These
+targets now require a complete selection for every parallel region while
+preserving partial updates for parallel states that are already active.

README.md

@@ -197,6 +197,11 @@ Transition contexts expose three typed target builders:
 | `target.branch` | Anywhere under the source's active top-level root | Replaces the selected branch while keeping omitted active ancestor values and parallel regions |
 | `target.full`   | Any top-level root                                | Builds a complete active snapshot for the selected root                                        |
 
+When `target.local` or `target.branch` enters an inactive nested parallel
+state, its callback must select every region, just like `initial` and
+`target.full`. When that parallel state is already active, `target.branch`
+can still update one region directly and preserves the other active regions.
+
 The builder controls how the next configuration is assembled; it does not by
 itself decide which invokes restart. The runtime derives exit and entry paths
 from the previous and next active paths. Shared active ancestors remain entered,

docs/agent-guide.md

@@ -170,7 +170,8 @@ const ParallelStates = Machine.defineStates({
 ```
 
 Every parallel region needs an active state in initial and full snapshot
-builders.
+builders. The same rule applies when a local or branch target enters an
+inactive nested parallel state.
 
 Use `type: "final"` for a terminal leaf in `Machine.defineStates`. A final
 child completes its compound parent. Put `onDone` on that completed parent,
@@ -208,6 +209,11 @@ until every declared output schema has an implementation.
 | `target.branch` | The destination is elsewhere under the active top-level root               | Omitted current ancestor values and parallel regions                              |
 | `target.full`   | The destination may be under any top-level root                            | Nothing is inferred for a newly selected root; build its complete active snapshot |
 
+Entering an inactive parallel state through `target.local` or `target.branch`
+requires a complete callback with one selection per region. A parallel state
+that is already active remains partially addressable through `target.branch`;
+unmentioned active regions are preserved.
+
 These describe configuration construction, not automatic process restart.
 Machine planning compares active paths and derives the actual exit and entry
 sets. A `target.full` result with the same active paths can update values without